The present invention relates to an apparatus for separating a dense phase from a light phase, and, in particular, to the cleaning of fluid media containing fine and/or ultra-fine particles.
Cyclone separators are known to be one of the least expensive means of dust collection from both an operating and an investment viewpoint, and is the most widely used type of dust collection equipment in the chemical and mineral process industries.
In cyclone separators, a gas laden with fine particles ("dust") enters through a tangential opening. The inertia of the fine particles tends to drive these particles toward the outside wall of the separator, from which they are led into a receiver and are subsequently discharged from the cyclone.
The vast majority of commercially-used cyclones are of the reverse flow (also known as indirect) cyclone in which the feed mixture containing the dust-laden fluid is typically introduced into the cyclone near the top end of the vessel, the flow being substantially tangential to the cylindrical portion of the cyclone. The fluid path involves a double vortex with the gas spiraling downward at the outside and upward at the inside. Upon entering the cyclone, he fluid velocity undergoes a redistribution so that the tangential component of velocity increases with decreasing radius. Having changed direction, the fluid phase moves in a counter-current manner relative to the dense phase moving along the outside wall of the separator.
Cyclones for removing solids from gases are generally applicable when particles of over 5 microns diameter are involved. Cyclones with very small diameters (and throughput) have been known to attain efficiencies of 80-85% on particles having a 3 micron diameter.
According to the "Chemical Engineering Handbook" (Perry and Chilton, Fifth Edition, McGraw-Hill, 1973), the collection efficiency in the removal of dusts can be changed by only a relatively small amount by a variation in operating conditions. The primary design factor that can be utilized to control collection efficiency is the cyclone diameter, a smaller diameter unit (operating at a fixed pressure drop) having the higher efficiency.
Soviet Union Patent No. 507,364 teaches a reverse cyclone separator for the separation of solid particles from a gas. The cyclone has a cylindrical-conical housing with a tangential inlet fitting, and an exhaust nozzle and a dust discharge port, both coaxial with the housing. The patent claims improved cleaning efficiency due to the fact that the diameters of exhaust nozzle and dust discharge port measure 0.4 and 0.2 of diameter D of the cylindrical part of the housing, respectively, while the cross-sectional area F.sub.in of inlet fitting equals to 0.06 of the diameter D of the cylindrical part of the housing squared. It is also indicated in the referenced patent that F.sub.in shall not be less than 0.055D.sup.2 and that he cyclone diameter to exhaust nozzle ratio D.sub.cycl /D.sub.ex shall not be greater than 2.5; otherwise, the cyclone resistance becomes exceedingly high.
According to the referenced description, a cyclone having a diameter of 400 mm provides a 95% efficiency when cleaning air from quartz dust with a median particle diameter of 14 microns.
A second, much less common type of cyclone is the co-current cyclone in which the exhaust nozzle and a dust discharge port are both coaxial with the housing, and the gas and dust-rich phase are discharged at the same end of the vessel in a co-current fashion.
U.S. Pat. No. 5,186,836 to Gauthier et al. discloses a co-current separator for the separation of a light phase from a mixture containing a light phase and a dense phase. Disposed downstream in the direction of circulation of the dense phase is an internal output opening of an interior enclosure. Fins are provided on the interior enclosure for limiting the progression of the light phase to the outside of the interior enclosure. The mixture is introduced tangentially into an inlet, and the dense phase is recovered at one outlet with the light phase recovered at another outlet. The co-current design makes it possible to rapidly separate the dense phase from the light phase.
The above-mentioned fins are intended to reduce the penetration of whirl from the separation chamber (a space between the cyclone cover and the output end of exhaust nozzle) into the bin, thereby reducing the reentrainment of fine particles from the bin.
According to U.S. Pat. No. 5,186,836, the recommended ratio of the cyclone diameter to exhaust nozzle output end diameter should be in the range from 1.7 to 2.5. The cyclone is designed to effect a very rapid separation between a dense phase and a light phase with a particularly narrow residence time distribution. The above-mentioned patent claims that the disclosed apparatus is effective in separating pellets with a diameter of 30 microns. It is manifestly evident to those skilled in the art that the above-mentioned cyclones do not provide an effective cleaning solution for fluids contaminated with particles in the range of 1-5 microns and in the sub-micron range.
One criterion for evaluating cyclone performance is the ratio of cleaning efficiency .eta.% to the cyclone resistance for the cleaning of unit volume of contaminated fluid from suspended particles of a given median diameter: ##EQU1##
where .eta.%=(1-C/C.sub.0)100%, and C.sub.0 are the suspended particle concentrations downstream and upstream of the cyclone correspondingly; W, the cyclone resistance, equals Q/.DELTA.P, where Q is the flow of fluid in m.sup.3 /s and .DELTA.P is the pressure drop of the apparatus.
The above equation shows that, given flow Q, two factors, the cleaning and the resistance, compete with one another. If the fluid to be cleaned represents gas, with the increase of dispersion of dust to be separated, the cleaning level of known cyclones rapidly decreases to 50-60% for 10-.mu.m dust and to as low as 1% for 1-3-.mu.m dust [A. K. Gupta et al., "Swirl Flows", Abacus Press (1984)].
Known cyclones are incapable of separating extremely fine dusts because the strong turbulence of flow greatly increases the effective flow viscosity. It is known that the effective viscosity in highly-turbulent flow regimes can be 10-100 times the viscosity of the same fluid in a laminar flow regime (Trans. Inst. Chem. Engrs., Vol. 51, 1973; Gupta et al.). The frictional forces in the cyclone, which are proportional to the effective viscosity, increase appreciably, resulting in a drastically reduced cleaning efficiency.
Moreover, such turbulence is a predominant factor behind the existence of a lower limit of cleaning in existing cyclones. It is known that the centrifugal force removing a particle from the flow is proportional to dust particle diameter cubed, d.sub.d.sup.3, while the friction force due to the radial component, is proportional to d.sub.d (Trans. Inst. Chem. Engrs., Vol. 51, 1973; Gupta et al.). As a result, with decreasing particle diameter, the centrifugal force decreases much faster than the frictional force, and the trajectories of the particles tend to approach the fluid streamlines.
However, it is also known that the centrifugal force is proportional to the tangential velocity squared, V.sub.t.sup.2, while the frictional force is proportional to the radial component of velocity, V.sub.r. Therefore, to improve the cleaning efficiency, the inlet (feed) flow velocity should be increased in order to increase the tangential velocity component. However, in existing cyclones, an increase in the inlet velocity results in increased turbulence. Because the frictional force also increases with increasing turbulence, there exists a high inlet velocity at which the increase in tangential velocity ceases to improve the cleaning efficiency due to increased frictional forces (resulting from the increased flow viscosity). Clearly, increasing the inlet velocity in existing cyclones beyond this point would only impair the cleaning quality. Thus, there is an inherent upper limit of cleaning in existing cyclones, such that high-efficiency separation of particles with a diameter of less than 5 microns is highly impractical or substantially impossible.
Moreover, the turbulence in existing cyclones severely decreases the tangential velocity component as compared with the ideal laminar flow case. In the ideal case, according to the angular momentum conservation law EQU rV.sub.t =constant,
V.sub.t increases in an inversely-proportional manner to the rotation radius r, attaining a maximum at the radius of the discharge nozzle (where the radius of the vessel is minimized). In practice, however, the inlet velocity (V.sub.in) of known cyclones is typically 20-25 m/s (with the tangential component of the inlet velocity virtually equaling V.sub.in due to the tangential inlet fitting), whereas at the discharge nozzle outlet, the tangential velocity does not exceed 3-4 m/s (Gupta, et al.), well below the theoretical value for laminar flow.
The cyclone power consumption is known to be proportional to the inlet and outlet velocities squared. The fraction of the input energy that is actually spent on cleaning is calculated by the ratio EQU V.sub.t.sup.2 /(V.sub.t.sup.2 +V.sub.in.sup.2),
wherein V.sub.t is the tangential component of velocity at the radius of the outlet and V.sub.in is the inlet velocity. Using this relationship, it can be seen that for known cyclones, the kinetic energy that is actually spent on cleaning amounts to only about 2% of the kinetic energy in the inlet flow (V.sub.in).
There is therefore a recognized need for, and it would be highly advantageous to have, a cyclone separator that provides improved separation of ultra-fine particulate matter, and more particularly, a cyclone separator that provides improved separation of ultra-fine particulate matter in a simple and cost-efficient manner.